"In the most severe cases, they don’t even know what music is," says Dennis Drayna, a communication disorders researcher at the National Institutes of Health. "It sounds like traffic noise to them."

Truly tone-deaf folks, who make up about 4 percent of the population, can't pick out differences in pitch or follow the simplest tune.

Amusics — people who cannot produce or distinguish musical sounds — have less dense white matter between their right frontal lobe, where higher thinking occurs, and the right temporal lobes, where we process sounds, than people with normal musical ability, according to a 2007 report in the Harvard Health Letter. People with the most severe cases of tone deafness had the thinnest white matter, which indicates a weaker connection between both parts of the brain.

If you think of the nerve connections as being like a highway, in tone-deaf people, there’s a lot less traffic traveling on the roadway than there is in people with normal musical abilities, said Beth Israel Deaconess Medical Center neurologist Psyche Loui, PhD, on the center's website. Dr. Loui's research has shown that the brain pathway that connects the perception and motor areas of the brain – known as the arcuate fasciculus – also has fewer connections among people who are tone deaf.

Amusics are able to function just fine aside from musical ability, but some studies show that they may have a hard time detecting some language cues.

Take the sentence "The boy went to the store." You read that differently from "The boy went to the store?" To a severe amusic, it might be hard to identify the difference based on tones alone, Drayna says.

Tone Deaf or Just Can't Carry a Tune?

The majority of people who might call themselves "tone-deaf" aren't actually amusics; they just have untrained ears. "I think that there are some people who lacked early music training who can't sing in tune, but with training they can get better." Some of these people don't know when they're singing out of tune, but many can hear music just fine. They just can't produce its, according to a 2011 study published online in the Journal of Experimental Psychology.

"If you weren't a great singer naturally while growing up, your choir director might have told you to sing softly so the rest of the group could drown you out," says Elizabeth Marvin, PhD, a professor of Music Theory at the Eastman School of Music at the University of Rochester. "You never got a chance to get better." Most anyone can improve with practice, she says.

As with any muscle, practice and regular use improves the strength of the muscles that control the vocal cords. And some tone-challenged people need time to produce music and hear themselves to understand how to hit the right note. Of course, practice won't turn us all into Mariah Carey or Josh Groban. There are limits to how much you can improve through repetition since producing music involves a complicated mix of skills from the ability to hear notes to voice control, Simone Dalla Bella, PhD, a psychologist at the University of Finance and Management in Warsaw told Live Science.

The Science of Perfect Pitch

There are a number of different theories out there about how people develop perfect pitch, the ability to produce a note on cue by name.

One idea, developed by Jenny Safran, PhD, a researcher at the University of Wisconsin, is that we're all born with some level of perfect pitch, but it eventually goes away. Think of it this way: babies can see a nose or mouth, but they aren't able to understand where to place them in relation to each other on a blank face. Similarly, cognitive processes that allow us to understand the tone of notes in relation to each other, or the relative pitch that most of us use, are advanced skills, compared to recognizing absolute pitch.

Other researchers say some people are born with a genetic predisposition to developing perfect pitch. For example, in a study of 73 families, researchers at the University of California, San Francisco, found a region of genes on chromosome eight in people with perfect pitch and from European ancestry.

Whether perfect pitch is universal or a genetic gift in children, most scientists agree that if you don't use the skill, you lose it. Music instruction in kids ages 4 to 6 is the key to preserving absolute pitch, the kind of musical gift that allow some adults to hear a note and immediately identify it as a B-flat. "It's almost always true that for adults with perfect pitch, they started with music instruction early in life," Dr. Marvin says. "If you have true absolute pitch, it’s with you for life."

Without that early training, it's almost impossible to develop that skill as an adult, Marvin adds. While you may be able to identify notes during intensive training, the ability easily gets lost without repeated use. "In adults trained in absolute pitch, it's effort-ful and it takes thought to decide, 'Oh, yeah, that must be an F sharp,'" Marvin says. "With true absolute pitch, it’s effortless. It’s like identifying a color.”